Traumatic brain injury (TBI) or neurotrauma contributes to numerous deaths and cases of permanent disability in the United States and world-wide. Of the 1.4 million people who sustain a TBI each year in the United States, 50,000 will die, 235,000 will be hospitalized, and another 1.1 million will be treated and released from an emergency department (Langlois J A, Rutland-Brown W, Thomas K E. Traumatic brain injury in the United States: emergency department visits, hospitalizations, and deaths, Atlanta (Ga.): Centers for Disease Control and Prevention, National Center for Injury Prevention and Control; 2004). Among children ages 0 to 14 years in the United States, TBI results in 435,000 visits to the emergency department each year, 2,685 fatalities, and 37,000 hospitalizations.
Diabetes mellitus is the most common cause of peripheral neuropathy in the United States (Kelkar, Seminars in Neurol, 2005; 25:168-173). Approximately half of all diabetics demonstrate symptoms of neuropathy. The usual clinical pattern is characterized by a slowly progressive mixed sensomotor and autonomic polyneuropathy. The presence of small-vessel disease in human diabetic nerves suggests that diminished endoneurial blood flow plays a role in human diabetic neuropathy, particularly with respect to scattered infarctions in the proximal regions of peripheral nerves attributable to vasa nervorum inflammatory vasculopathy (Singleton, Seminars in Neurol, 2005; 25:185-195).
Abnormal spiking activity and peripheral neuropathy excitotoxic activation of pre- and postsynaptic NMDA receptors is a key event that triggers downstream pathways implicated in subsequent neuronal death in cases of cerebral ischemia underlying neurotrauma and stroke. Peripheral neuropathy mostly causes damage in pre-synaptical membranes while post-synaptical events imply cerebrovascular system impairment.
The N-methyl-d-aspartate subtype of glutamate receptor (NMDAR or NMDA receptor) serves critical functions in physiological and pathological processes in the central and peripheral nervous system, including neuronal development, plasticity and neurodegeneration. Various investigators have reported that the receptor consists of three primary subunits: NR1, NR2A-D, and NR3A-B, and that the coexpression of NR3A with NR1 and NR2 subunits modulates NMDAR activity in pre- and post-synaptical membranes.
The postsynaptic NR2 subfamily consists of four individual subunits, NR2A to NR2D. In situ hybridization has revealed overlapping but different expression for NR2 mRNA. In particular, NR2A mRNA is distributed ubiquitously like NR1 with highest densities occurring in hippocampal regions. In contrast, NR2B is expressed predominantly in the forebrain but not in the cerebellum where NR2C predominates (Parsons et al., Drug News Perspect. Nov. 1998; 11(9):523-569). The spinal cord reportedly expresses high levels of NR2C and NR2D (Tolle et al., J Neurosci. 1993 December; 13(12):5009-28) and these may form heteroligomeric receptors with NR1 plus NR2A (Sundstrom et al., Exp Neurol. 1997 December; 148(2):407-13).
NR3 is reportedly expressed predominantly in the developing central nervous system and does not seem to form functional homomeric glutamate-activated channels (Sucher et al., J Neurosci. 1995 October; 15(10):6509-20). From in situ and immunocytochemical analyses, it is known that NR3B is expressed predominantly in motor neurons, whereas NR3A is more widely distributed.
Zukin et al. have reported that alternative splicing generates eight isoforms for the pre- and postsynaptic NR1 subfamily (Zukin and Bennett, Trends Neurosci. 1995 July; 18(7):306-13). The variants arise from splicing at three exons; one encodes a 21-amino acid insert in the N-terminal domain (N1, exon 5), and two encode adjacent sequences of 37 and 38 amino acids in the C-terminal domain (C1, exon 21 and C2, exon 22). NR1 variants are sometimes denoted by the presence or absence of these three alternatively spliced exons (from N to C1 to C2). NR1111 has all three exons, NR1000 has none, and NR1100 has only the N-terminal exon. The variants from NR1000 to NR1111 are alternatively denoted as NMDAR1e, c, d, a, g, f, h and b respectively or NMDAR1-4a, -2a, -3a, -1a, -4b, -2b, -3b and -1b respectively, but the more frequent terminology uses non-capitalized subscripts, which suffices for the most common splice variants, i.e. NR1a (NR1011 or NMDAR1A) and NR1b (NR1100 or NMDARIG). NR1a receptors are more concentrated in rostral structures such as the cortex, caudate, and hippocampus, while NR1b receptors are principally found in more caudal regions such as the thalamus, colliculi, locus coeruleus and cerebellum (Laurie et al., Brain Res Mol Brain Res. 1995 August; 32(1):94-108).
The role of NMDA receptors has been explored by numerous investigators. For example, it has been reported that the process of peripheral and central sensitization is maintained, at least theoretically and experimentally, through the excitatory neurotransmitter glutamate, which is believed to be released when the NMDA receptor is activated (Gudin, Medscape Neurology & Neurosurgery 2004). In addition, available evidence suggests that the roles of NMDA receptors differ with respect to the processing of visceral and somatic pain. One set of authors have concluded that NMDA receptors are present in peripheral visceral nerves and may be important in visceral pain processing in the absence of inflammation, thus providing a novel mechanism for development of peripheral sensitization and visceral hyperalgesia (McRoberts et al., Gastroenterology 2001; 120:1737-1748).
In a number of studies, blocking NMDA receptors has been proposed as a preventive treatment for protecting neurons from ischemia (Dugan L L and Kim-Han J S In:Basic Neurochemistry. Siegel et al. Eds, 2006, 7th edition, 559-73). However, blocking NMDA receptors may be detrimental to animals and humans (Davis et al, Stroke 2000; 31:347-354; Ikonomidou et al, Proc. Natl. Acad. Sci. U.S.A. 2000; 97:12885-12890). Moreover, although blocking excitotoxicity of NMDA receptors has proven effective in laboratory models of disease, clinical trials of neuroprotective therapies have generally failed to benefit patients (Lees et al. (2000) Lancet 355:1949-1954). These failures are generally attributed to side effects of glutamate receptor antagonists which may evoke failure of high brain functions (mental disturbances, memory decline and asocial behavior).
Some limited efforts have been made at using natural peptides derived from the brain for treating cerebral ischemic events (Gusev, Skvortsova. Brain Ischemia. NY-Boston-Dordecht-London: Kluwer Academic/Plenum Publishers, 2003; 382). For example, it has been shown in clinical trials that ACTG hormone 4-10 fragment (Semax) drastically improves movement and mental performance in patients who have suffered an acute stroke. Cerebrolyzin, an extract of small peptides from pig brain, has shown positive clinical effect optimizing energetic metabolism of nervous cells and Ca2+ homeostasis. It has also been shown that cerebrolysin in a dose of 10 mg daily reduces lipid peroxidation and the accumulation of glutamate receptor antibodies, thereby improving patient memory, speech and psychological activity (www.consilium-medicum.com).
Recently, NMDAR peptides and their antibodies have been proposed for the treatment of stroke and epilepsy (During et al, Science, 2000, 287:1453-60) and as biomarkers of neurotoxicity underlying cerebral ischemia and stroke (Dambinova S A, et al. Stroke 2002; 33:1181-1182; Dambinova S A, et al. Clin Chem 2003; 49:1752-1762). With neuronal death or ischemia, NR2 peptide fragments of the NMDA receptor break off and appear in the bloodstream and generate an antibody response. Dambinova et al, have reported that the peptide fragments and antibodies can both be detected in blood samples (Dambinova S A, et al. Stroke 2002). They have further reported that adult patients who have suffered an acute ischemic stroke have elevated blood levels of NR2 peptide/Ab that correlate with the amount of brain damage revealed through brain scans (MRI) and neurocognitive testing (Dambinova S A, et al. Clin Chem 2003; 49:1752-1762).